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Because the theory of evolution does a lot of damage to a literalist interpretation of the bible, many believers and various conservative Christian denominations are reluctant to accept it as valid, even though among scientists relevant to the field, evolution is a settled matter. It is a scientific fact. (If you’re hung up on the term “theory”, as it pertains to evolution, you should know that when scientists use the word “theory”, it has a different meaning from how the word is used in normal everyday conversation. In scientific usage, the term "theory" is reserved for explanations of phenomena which meet basic requirements about the kinds of empirical observations made, the methods of classification used, and the consistency of the theory in its application among members of the class to which it pertains. See:Scientific theories).

There is plenty of evidence that supports the theory evolution, such as the very fact that disease-causing micro-organisms evolve to become resistant to antibiotics, or how certain pests evolve to become resistant to certain pesticides. Animal breeders have had centuries of experience selecting certain desirable traits in some domestic animal breeds and having them mate with others to produce off-spring bearing the said desirable traits, or sometimes hybrids that may share traits of both. Creationists (i.e. people who reject evolution on the grounds of their belief that all living organisms were created by a ‘God’) normally concede this point, but dismiss it on the basis that it is “micro” evolution, and then go on to insist that “macro” evolution is what is false. Here is an excerpt from an essay called “Microevolution Doesn’t Prove Macroevolution” from the United Church of God website, expounding upon this kind of objection:

Studies that find small variations within a species over time, such as in the size of finch beaks or the coloration of moths, are sometimes used to try to prove Darwinian evolution. But such studies are sometimes flawed. And even if valid, they provide no such proof.

Adaptation within a species is called microevolution. It is the same phenomenon at work when the average height of men and women increased by several inches in the Western world over the course of the 1900s. Better health and nutrition played a large part in producing larger-sized people. In the same way, microevolution is at work when breeders produce varieties ranging from Chihuahuas to Great Danes within the one species Canis familiaris —the domestic dog.

These examples show, as in the rest of nature, that all species do have a margin of change available within their genetic pool to adapt to conditions. This trait is found in man, who can adapt to freezing weather, as the Eskimos do, or to the broiling sun in the desert, as bedouins have done. But bedouins and Eskimos are still human beings, and if they changed environments again, eventually their offspring would also go through minor changes to better adapt to their new environment.

What has never been scientifically demonstrated—in spite of many examples of wishful thinking—is macroevolution, or the change from one distinct species to another. Dogs have never evolved into birds or human beings.

But is this objection a valid one?

No, it is not:

The term "macroevolution" frequently arises within the context of the evolution/creation debate, usually used by creationists alleging a significant difference between the evolutionary changes observed in field and laboratory studies and the larger scale macroevolutionary changes that scientists believe to have taken thousands or millions of years to occur. They may accept that evolutionary change is possible within species ("microevolution"), but deny that one species can evolve into another ("macroevolution"). Contrary to this belief among the anti-evolution movement proponents, evolution of life forms beyond the species level ("macroevolution", i.e. speciation in a specific case) has indeed been observed multiple times under both controlled laboratory conditions and in nature. The claim that macroevolution does not occur, or is impossible, is thus demonstrably false and without support in the scientific community.

Such claims are rejected by the scientific community on the basis of ample evidence that macroevolution is an active process both presently and in the past. The terms macroevolution and microevolution relate to the same processes operating at different scales, but creationist claims misuse the terms in a vaguely defined way which does not accurately reflect scientific usage, acknowledging well observed evolution as "microevolution" and denying that "macroevolution" takes place. Evolutionary theory (including macroevolutionary change) remains the dominant scientific paradigm for explaining the origins of Earth’s biodiversity. Its occurrence is not disputed within the scientific community. While details of macroevolution are continuously studied by the scientific community, the overall theory behind macroevolution (i.e. common descent) has been overwhelmingly consistent with empirical data. Predictions of empirical data from the theory of common descent have been so consistent that biologists often refer to it as the "fact of evolution".

In science, macro at the beginning of a word just means "big", and micro at the beginning of a word just means "small" (both from the Greek words). For example, "macrofauna" means big animals, observable by the naked eye, while "microfauna" means small animals, which may be observable or may not without a microscope. Something can be "macro" by just being bigger, or there can be a transition that makes it something quite distinct.

In evolutionary biology today, macroevolution is used to refer to any evolutionary change at or above the level of species. It means at least the splitting of a species into two (speciation, or cladogenesis, from the Greek meaning "the origin of a branch", see Fig. 1) or the change of a species over time into another (anagenetic speciation, not nowadays generally accepted [note 1]). Any changes that occur at higher levels, such as the evolution of new families, phyla or genera, are also therefore macroevolution, but the term is not restricted to those higher levels. It often also means long-term trends or biases in evolution of higher taxonomic levels.

Former Professor for Public Understanding of Science at Oxford University, Richard Dawkins, makes a case for evolution in this interview:

In the video below, biologist Kenneth Miller talks about the relationship between Homo sapiens and the other primates. He discusses a recent finding of the Human Genome Project which identifies the exact point of fusion of two primate chromosomes that resulted in human chromosome #2:

To view Kenneth R. Miller’s full lecture (1hr 58min 42sec): The Collapse of Intelligent Design, go here. (Kenneth Miller is a Roman Catholic, by the way)

A Nice Analogy:

I came across a rather simple and interesting way of explaining to creationists HOW macroevolution works. I don’t know who first came up with it, but I found it via Reddit. It goes like this: